Volatile organic compounds (VOCs) as described by the United States Environmental Protection Agency (EPA) include components of fuels, solvents, and chemical feedstocks commonly used for internal combustion engine fuel, power and heat generation, cleaning, chemical, pharmaceutical, agricultural, semiconductor and other industries. VOCs are highly regulated in the U.S. and elsewhere in the world because they contribute to photochemical smog formation. A subset of VOC compounds includes those compounds designated by the EPA as toxic chemicals, including those compounds designated as Air Toxics. “Air Toxics” are harmful to breathe. As such they are regulated by the EPA in ambient and indoor air, and by OSHA in the workplace.
Atmospheric VOCs and/or Air Toxics are currently measured under USEPA guidance at regular times and places as part of the Photochemical Assessment Monitoring Stations (PAMS). These VOCs may be measured according to EPA Method TO-14A using samples collected in special canisters. Another method for measuring Air Toxics or VOCs uses active sampling into sorbent tubes using EPA Method TO-17. In either case the canisters or sorbent tubes are then transported to a gas chromatography laboratory for analysis using (for instance) thermal desorption of the adsorbent cartridges, or flushing or pumping from the canisters. This is followed by cryogenic or some other type of cooling. Detailed instructions on these procedures are freely available from the USEPA, which publishes the TO-xx methods. Gas chromatography methods for air analysis are recently summarized in an extensive review article written by Detlev Helmig, entitled “Air Analysis by Gas Chromatography,” Journal of Chromatography A, 843:129-146 (1999).
Harmful or toxic chemicals based upon organic chemicals typically have a carbon skeleton and usually are derived from petroleum. The simplest members of this wide range of compounds are hydrocarbons (HC's), compounds containing only the elements carbon and hydrogen. Hydrocarbons consist of alkanes (all single bonds), alkenes (at least one carbon/carbon double bond), alkynes (at least one carbon/carbon triple bond), and aromatics, which contain conjugated carbon/carbon double bonds, and are derivatives of benzene, C6H6. These bonding functionalities may exist in combination with one another, making an individual hydrocarbon belong to more than one class. There is no strict upper limit to the molecular weight, molecular size, or carbon number of such compounds. As the carbon number increases, the compounds have decreasing vapor pressure and, if present in the atmosphere at all, are increasingly present in suspended particulate matter rather than as gases. Table I provides exemplary members of each HC family.
TABLE IExamples of Hydrocarbons Classified as VOCsAlkanesAlkenesAlkynesAromatics1.Methane1.Ethene1.Ethyne1.Benzene(CH4)(C2H4)(C2H2)(C6H6)2.Ethane2.Propene2.Propyne2.Methylbenzene(C2H6)(C3H6)(C3H4)(C7H8), i.e.,toluene3.Propane3.Butene3.Butyne3.Ethylbenzene(C3H8)(C4H8),(C4H6),(C8H10)whichwhichexists inexists inisometricisomericformsforms4.Butane4.Butadiene4.Dimethylbenzene(C4H10),(C4H6)(C8H10),whichi.e., xylene,exists inwhich exists inisomericisomeric formsforms5.Naphthalene(C10H8)
Other VOC compounds include carbon, hydrogen, and at least one other element, especially including (but not limited to) the elements oxygen, sulfur, nitrogen, phosphorus, and the halogens, such as fluorine, chlorine, bromine and iodine. Such compounds are used in the chemical, electronics, agricultural, and many other industries as solvents, pesticides, drugs, and so forth. Compounds containing the elements C, H, and O are sometimes called oxygenated volatile organic compounds, OVOCs. Table II provides a few exemplary members of this extended VOC family.
TABLE IIExamples of VOCs other than Pure HydrocarbonsOxygen-ContainingOVOCsSulfur-ContainingHalogen-Containing1.Aldehydes1.Sulfides1.Chlorocarbons (e.g., CHCL3,CH2Cl2, CH3Cl, CH3CCl3,C2Cl42.Ketones2.Sulfates2.Halons (e.g., CH3Br, CH3I)3.Acids3.Mercaptans3.Chlorofluorocarbons (e.g.,CCl2F2, CClF3, CCl4, CF4)4.Ethers4.Thiols5.Alcohols
A partial listing of chemical compounds found in the atmosphere is Chemical Compounds in the Atmosphere, (1978) Academic Press, T. E. Graedel. This book lists many hundreds of such compounds known when it was published more than 20 years ago.
A major source of atmospheric hydrocarbons is automobile gasoline, which typically contains hydrocarbons having carbon numbers greater than 3. Methane, natural gas, is widespread and relatively constant in the atmosphere at concentrations of about 1.8 ppm by volume. Natural gas is about 95% methane and 5% ethane. Propane makes up the bulk of liquefied petroleum gas (LPG). Gasoline and diesel fuel and their resulting combustion byproducts together contain more than 200 individual hydrocarbons. See Fraser et al., “Air Quality Model Data Evaluation for Organics. 4. C2-C36 Non-aromatic Hydrocarbons,” Environ. Sci. Technol., 31:2356-2367 (1997). Since these compounds, along with oxides of nitrogen also produced in combustion, react chemically in the atmosphere to produce smog, there is worldwide interest in controlling their atmospheric emission, and in measuring their individual (speciated) concentrations.
Air Toxics are compounds directly harmful to human health, and the EPA has many regulations dealing with their emission and atmospheric concentration. Efficient measuring of ambient concentrations is highly important. All ambient gaseous compounds also appear in human breath since they are inhaled. In addition, metabolic processes add additional volatile compounds to exhaled breath, such as ethanol, acetone, isoprene, pentane and others. Study of metabolic processes of respiratory organisms and diagnosis of disease would benefit greatly from automated VOC analysis in exhaled air. Chromatographic analysis of anesthesia environments such as hospitals has been reviewed by A Uyanik in Journal of Chromatography B 693 (1997) 1-9. From this review it is clear that a sensitive, inexpensive, compact gas chromatograph would be a useful tool for operating rooms and associated environments.
Sick Building Syndrome involves poorly characterized human diseases and ailments associated with outgassing of toxic materials in the indoor environment. Sources of such toxic materials can include carpets, drapes, particle board, etc. Harmful fungi and bacteria which can thrive in moist or poorly ventilated environments often emit characteristic VOC or OVOC compounds (e.g. heptanol) which, although they may not be toxic themselves, can serve as indicators of the presence and abundance of such harmful organisms.
Chemical synthesis or process streams, clean rooms and other industrial areas require automated, sensitive gas analysis procedures which may be routinely implemented for reasonable costs. Other areas which would benefit from highly sensitive analytical air analysis methods would be those areas dealing with naturally occurring and artificially applied pheromones for insect attraction and/or control.
In sampling trace level VOCs, air toxics, metabolites or other analytes such as particulates in the atmosphere, in breath, or other gaseous environments, the concentration of target analytes often is below the detection limit of a particular analytical technique. Such analysis is often termed trace gas analysis. A wide range of concentrations may be present, for instance from 1 ppmV (1 part per million by volume) down to 1 pptV—a range of one million. For instance, in gas chromatography, a flame ionization detector cannot detect many VOCs of ambient atmospheres or in breath samples unless they are concentrated. Two concentration methods are commonly employed: (a) cryogenic focusing/concentration and (b) adsorbent focusing/concentration. In each method an air sample of the desired volume is passed through an accumulation chamber, which consists of:                (a) a ‘U-tube’ immersed in a cryogenic liquid, such as liquid oxygen or air, or which is otherwise cooled sufficiently that some or all of the target analytes condense to liquids or solids within the U-tube trap, also referred to herein as a cryotrap. Most of the air sample does not condense and therefore passes through the trap; or        (b) a sorbent-filled trap, which absorbs or adsorbs some or all of the target analytes, allowing most of the sample to pass through. Such traps can operate at ambient temperature or below.        
Either procedure concentrates the desired analytes to a concentration much higher than their original concentration in the air sample. After the desired air volume has passed through the trap, yielding sufficient analyte, the trap is heated to transfer the concentrated analytes into a chromatographic column or other analytical device.
Both of these procedures are commonly used in the field of atmospheric analysis, air pollution, etc. However, each has drawbacks, which makes them less amenable to automating an air-monitoring instrument, especially for field use. In the case of cryogenic focusing, the cryogenic liquid must be stored on site and pumped as needed for cryogenic focusing. Although electrically cooled devices are available, such devices typically cannot obtain sufficiently low temperatures to collect all of the VOCs that can be condensed by cryogenic focusing. Another problem with cryofocusing is the large amount of atmospheric materials, particularly water and carbon dioxide, which are trapped along with desired analytes, unless separately removed before the cryotrap. Yet another problem with cryofocusing is that such instruments typically reside in laboratories to which samples must be transported in special containers. Although such transport has been extensively studied, there remains the possibility of sample modification so that spurious compounds may either be added to or subtracted from transported and/or stored samples. For sorbent-filled traps, the sorbent material must adsorb and desorb a wide range of potential analytes because the target analyte volatilities vary greatly. A strongly absorbent material may collect all analytes, but temperatures high enough to cause desorption of the least volatile analytes may cause decomposition of analytes or the absorbent collecting material itself. A less absorbent material may sorb and desorb the heavier analytes, but not collect the more volatile analytes, which therefore are not completely collected. Another problem with sorption is the tendency for the material to desorb over a period of time when heated. This can require refocusing with cryogens or other methods during analysis. Sorbent and cryofocusing can be used in combination. A final problem with adsorbents is possible chemical reaction or decomposition of the target analytes during collection, transport or storage of the adsorbent cartridges, or the presence of artifacts acquired on the adsorbents before or after sampling. Such artifacts are not uncommon in atmospheric sampling and often lead to spurious conclusions about atmospheric trace-gas composition. Ambient air sampling and breath analysis would benefit greatly from in-situ, continuous, real time analytical instrumentation. Such instrumentation is not widely available nor currently practical.
Gas chromatography methods for air analysis are recently summarized in an extensive review article written by Detlev Helmig, entitled “Air Analysis by Gas Chromatography,” Journal of Chromatography A, 843:129-146 (1999), which is incorporated herein by reference. Helmig's review substantiates the conclusion that only two primary methods are known for concentrating analytes in an ambient air sample, cryofocusing and absorbent traps. These methods are poorly amenable to developing remotely operated, continuous sampling methods for ambient air although such methods have been reported. For instance J P Greenberg, B Lee, D Helmig and P R Zimmerman have described a “Fully automated gas chromatograph-flame ionization detector system for the in situ determination of atmospheric non-methane hydrocarbons at low parts per trillion concentration” in Journal of Chromatography A 676 (1994) pp. 389-98. This system was designed to (1) rapidly trap air samples of up to 4 liters volume to allow for sub-parts per trillion detection limits, (2) eliminate interferences from ambient ozone, water vapor and carbon dioxide, and (3) reduce to negligible levels any contamination in the analytical systems, and (4) allow for continuous unattended operation. This instrument used cryogenic sample freeze-out and was successfully employed for measurements in the state of Hawaii. However, it apparently has seen limited additional use since that time, probably because of its cost, complexity and use of cryogenic fluids.
Other pertinent areas include breath analysis. For instance, U.S. Pat. No. 5,293,875, “In-vivo Measurement of End-tidal Carbon Monoxide Concentration Apparatus and Methods” describes a noninvasive device and methods for measuring the end-tidal carbon monoxide concentration in a patient's breath, particularly newborn and premature infants. The patient's breath is monitored. An average carbon monoxide concentration is determined based on an average of discrete samples in a given time period. An easy to use microcontroller-based device containing a carbon dioxide detector, a carbon monoxide detector and a pump for use in a hospital, home, physician's office or clinic by persons not requiring high skill and training is described.
K D Oliver and 7 co-authors of Mantech Environmental, the USEPA, XonTech and Varian Chromatographic Systems have described a “Technique for Monitoring Toxic VOCs in Air: Sorbent Preconcentration, Closed-Cycle Cooler Cryofocusing and GC/MS analysis” in Environmental Science and Technology 30 (1996) 1939-1945. This powerful but very complex, automated system usually is attended by various operators and has seen only intermittent field use, perhaps due to operational expense and complexity.
Air pollution is increasingly regulated throughout the world. Knowing the source of pollution emissions is essential to this regulatory process so that regulation can be efficient and cost effective. One method for determining air pollution sources is source characterization. That is, individual sources are surveyed either by direct measurements of emissions or by apportionation by generic emission factors. Usually local, regional, or national pollution control agencies maintain emission inventories and issue emission permits. Such emission inventories are widely viewed as unreliable. Once emission factors for a variety of pollutant species, including VOCs, are available, individual measurements of atmospheric VOCs at any site can be assigned quantitatively to the major sources by mathematical processes referred as Source Apportionment or Chemical Element Balances. Efficient, cost-effective measurements of ambient VOCs, Air Toxics, and other pollutant concentrations will allow this source apportionment procedure to be carried out more efficiently. Beyond source apportionment, recently developed computer programs (program UNMIX developed by Dr. Ronald Henry of the University of Southern California) now allow sources to be determined from ambient VOC measurements without any direct source information. (See Science News Online Jun. 28, 1997 and the USC Chronicle Sep. 1, 1997, included herein) As Dr. Henry describes it, these programs allow the ambient air data to analyze itself This extremely powerful new mathematical technique would benefit greatly from low-cost, and therefore frequent measurements of VOCs and other such compounds in polluted air.
In addition to the organic compounds discussed above, there is a need for the determination of various inorganic atmospheric constituents. A few examples are NO, NO2, SO2, H2S, O3, CO, etc. Many of these have specific instrumental methods and measurement devices devoted specifically to their determination, for instance in automobile testing as well as in ambient air. A more general method involves measurement of one or more of such species (including VOC and OVOC compounds discussed above) by light absorption. This may occur typically in the ultraviolet, visible, or infrared. When species are present at very low concentrations, often long path lengths are used. This may involve meters or kilometers through the open atmosphere, or reflected paths in a localized instrument. Examples of such techniques are differential optical absorption spectroscopy (DOAS) and Fourier transform infrared spectroscopy (FTIR). Such instruments may determine one or many atmospheric components simultaneously using light at various suitable wavelengths.
Despite these previously developed techniques and inventions, there still is a need for an apparatus and method for continuous, and remote if desired, concentration and analysis of gaseous samples. Such a method and apparatus, if available, would allow automation of methods for analyzing analytes in a gaseous sample, such as air-pollution analysis, clinical breath analysis, metabolic studies, process streams, clean rooms, etc.